The general rule is one finger per fret per position. The rules get broken as needed.
Position is just the fret number that your fretting hand index finger is positioned at. If it's at the 5th fret, it's 5th position; 7th fret, it's at 7th position, etc, etc.
For example: If you are in 2nd position, your index finger is at the second fret and will play all the notes located on the 2nd fret; your middle finger will be on the 3rd fret and it will play all the notes located on the 3rd fret; your ring finger will be on the 4th fret and will play all the notes located on the 4th fret; finally, your pinky will be on the 5th fret and will play all the notes located on the 5th fret.
Again, this is just the general guideline subject to exceptions and the choice of the player. If Griff does suggest a fingering in the video, then I would suggest you give a try and stick with it for now, even if it seems hard. There is usually an advantage of one fingering over another if he takes the time to point it out and you will probably benefit from learning it.